Abstract Function and ligands of orphan receptor GPR50 We propose to identify agonists of the orphan melatonin-related G-protein coupled receptor 50, (GPR50), such compounds may provide a starting point for the development of therapies for bipolar disorder. In Phase I of this project, we will develop functional assay systems for GPR50 and conduct preliminary screens using a focused GPCR library. Many GPCRs have no known endogenous ligand or function. These orphan GPCRs may provide major opportunities in the development of new drugs to better treat common and pervasive mental disorders. Modulators of GPR50 have potential for treating bipolar disorder. GPR50 is a member of the melatonin receptor family. Despite 74% identity with the melatonin receptor, it is unable to bind melatonin and the endogenous ligand of GPR50 is not known. Using its proprietary expression vectors, Multispan Inc. has developed cellular expression systems and assays for numerous GPCRs including orphans using mammalian cell lines. We will carry out the following aims: Aim 1. Generate cell lines expressing GPR50 in a stable HEK293T cell cline containing promiscuous G-protein subunit Galpha16, a promiscuous adaptor protein that funnels GPCR signal transduction into a common pathway via phospholipase Cbeta and the release of calcium ions from intracellular stores. Aim 2. Confirm GPR50 activity. Aim 3. Assay for agonist activity using GPCR-focused library. In Phase II we will examine ligand binding in multiple disease-linked polymorphisms of GPR50, screen a larger GPCR- focused library in our assay and test identified hits in animal models of behavioral neuropathologies. PUBLIC HEALTH RELEVANCE Function and ligands of GPR50 The goal of this Phase I SBIR project is to find a ligand to GPR50, a cell membrane protein in the GPCR family. GPR50 is found in the hypothalamus and genetic studies have linked this protein to bipolar disorder although its function is unknown. New drugs for bipolar disorder are urgently needed, as current drugs are not effective in many patients or have adverse effects. Multispan Inc. routinely makes mammalian cells that express functional GPCRs for screening assays, and will apply this proven approach for GPR50. These cells will be tested against a collection of chemicals targeted to GPCRs at the University of California, Irvine, using its High Throughput Screening service. Our long term goal is new drugs for bipolar disorder. [unreadable] [unreadable] [unreadable]